Radiating transmission lines are deliberately constructed as imperfect transmission lines so that signals in the inner conductor radiate electromagnetic fields outwardly from the line as the electrical signals are being transmitted down the line. The electrical magnetic fields radiated from the line can be picked up by mobile receivers located remotely, but in the vicinity, of the transmission line. Furthermore, components can be connected directly to the radiating transmission line to either modify the signal, such as amplifying it, branch the signal to more than one line, or receive and send communication signals such as through radio phones and other electrical components. Radiating transmission line communication systems may also transmit data and/or video signals as disclosed, for instance, in U.S. Pat. No. 5,697,067 to Graham et al. assigned to the same assignee as the present application and incorporated herein by reference.
Radiating transmission lines can take on several different forms. One form comprises an open braid coaxial cable. Other forms comprise coaxial cables having cylindrical outer sheaths with longitudinal slits to permit radiation.
Radiating transmission lines are commonly used in environments where electromagnetic waves, such as radio frequency waves, do not propagate well. This type of environment exists, for example, in underground mine shafts. For example, a worker in a mine shaft using a remote mobile communication unit, such as a mobile radio or walkie-talkie, cannot communicate to other workers who also have remote mobile communication units because the radio waves cannot propagate long distances down a mine shaft. However, if all of the workers are near a radiating transmission line, such that the radio waves from the first worker's remote mobile communication unit could be received by the transmission line, those signals could be transmitted by the radiating transmission line, modified and then radiated near the remote mobile communication units of other workers. In this way, communication in a mine shaft or other environment where radio frequency waves do not propagate well, can be effected.
In the past, several different types of communication systems utilizing radiating transmission lines have been used. However, a common difficulty with most of the prior art communication systems has been that a degradation in the system, such as a fault in the radiating transmission line or a component in the system, effectively prevents communication to the portion of the mine downstream from the fault. For example, if a degradation in transmission arose at a point in the communication system such that a communication signal could not travel on the transmission line, either into or out of the mine, then persons located downstream of the degradation and going deeper into the mine could not communicate with others.
Such a degradation in the communication could arise from a fault in the radiating transmission line itself, for example, if the radiating transmission line was cut, developed a short circuit or was damaged such as by way of water, fire or an explosion. A fault could also arise in the electronic circuitry of one of the electronic units, such as an amplification unit or a branch unit. A malfunction in one of the mobile radios would not necessarily be considered a fault in the system, because only the mobile radio affected would cease to operate and the other elements of the system would continue operating as before, and therefore such a fault would not typically result in degradation of transmission of communication signals in the communication system.
A radio frequency communication system for overcoming faults is also disclosed in U.S. Pat. No. 5,669,065 to Patrick M. Y. Waye and Kenneth John Morrell, which has been assigned to the same assignee as the present application and is incorporated herein by reference. U.S. Pat. No. 5,669,065 discloses a system for overcoming a fault in a communication system where the radiating transmission line loops around the underground environment either directly or through branching units, such that both the first end and the second end of the transmission line may meet at the base station. When a fault occurs in the system disclosed in U.S. Pat. No. 5,669,065, the base station commences transmitting and receiving signals from both ends of the transmission line and the amplification units are reconfigured to communicate signals away from the fault and towards the base station. While U.S. Pat. No. 5,669,065 is very effective in mines having two connecting bores, such that the transmission line can loop around the two connecting bores and both ends of the transmission line may meet at the base station, the communication system disclosed in U.S. Pat. No. 5,669,065 would not operate well in cases where the mine has a single bore, or has a second bore that intersect the first bore, or if there are multiple faults isolating sections of the radiating transmission line. In such a case, faults occurring in the communication system could render communication inoperable to locations between the fault and termination units located downstream from the fault.
U.S. Pat. No. 5,278,989 to Burke discloses a communication system for serving a plurality of radio units in a frequency blocked area, and, comprising radiating transmission lines and amplification units associated with the cables. In the event of a fault in one of the application units being detected, control data is passed along the network of radiating cables for changing the path of communication through the communication system. This system provides for a central control station to communicate with each amplifier unit to assist in detecting faults in the amplifiers, but does not disclose a means for detecting faults or degradations along the transmission lines. Upon detection of a fault in the amplification unit, the system disclosed in U.S. Pat. No. 5,278,989 sends a signal to the amplification unit experiencing the fault, to reconfigure the amplifier which is experiencing the fault, such that energy received over the coaxial cable is amplified by a secondary or backup amplifier in the amplification unit rather than the primary amplifier. However, the system disclosed in U.S. Pat. No. 5,278,989 would not overcome a degradation in the communication system which occurred, for example, by damage to the transmission line as opposed to a fault in the amplifier. Rather, U.S. Pat. No. 5,278,989 merely discloses use of a secondary or backup amplifier which is activated upon detecting a fault in the primary amplifier.